1. Field of the Invention
The present invention relates to a power converter, and more specifically relates to a control circuit of the switching power converter.
2. Description of Related Art
Power converters have been widely used for providing regulated outputs. For safety, the power converter must provide the isolation between its primary side and secondary side. A transformer is usually equipped to provide the isolation and the energy transfer. FIG. 1 shows a power converter having a transformer 10. The energy is stored into the transformer 10 during a switch 20 is on, in which the switch 20 is a power transistor or a power MOSFET. The energy will be discharged to the output of the power converter when the switch 20 is switched off. A current-sense resistor 21 is connected in series with the switch 20 to sense the switching current of the transformer 10 and generate a current signal VS for the switching control. A voltage-detection terminal DET and a current-sense terminal VS of a controller 25 are respectively coupled to the transformer 10 and the current-sense resistor 21 to generate a control signal VG to control the on/off of the switch 20 and regulate the output of the power converter. A compensation terminal COM of the controller 25 connects a compensation capacitor 24.
The transformer 10 includes a primary winding NP, a secondary winding NS and an  auxiliary winding NAUX. The secondary winding NS connects a rectifier 15. A filter capacitor 17 is coupled between the rectifier 15 and the secondary winding NS. Once the switch 20 is turned off, the auxiliary winding NAUX will generate a flyback voltage VF correlated to the output voltage VO. The flyback voltage VF can thus be utilized to feedback the output voltage VO. A flyback control technique has been disclosed in U.S. Pat. No. 4,302,803 “Rectifier-Converter Power Supply with Multi-Channel Flyback Inverter”, issued to Randolph D. W. Shelly. However, the disadvantage of the foregoing prior art is the flyback voltage cannot be accurately measured, particularly at the light load condition. FIG. 2 shows the waveforms of the power converter at light load. The discharge time TDS of the transformer 10 is given by,
                              T          DS                =                              (                                          V                IN                                                              V                  O                                +                                  V                  D                                                      )                    ×                                    W              NS                                      W              NP                                ×                      T            ON                                              (        1        )            where VIN is the input voltage of the power converter; WNP and WNS are respectively the winding turns of the primary winding NP and the secondary winding NS of the transformer 10; VD is a forward voltage drop of the rectifier 15; TON is an on-time of the control signal VG.
The flyback voltage VF is connected to the voltage-detection terminal DET of the controller 25 through a resistor 22. A parasitic capacitor 23 is coupled to the resistor 22. A detecting voltage VDET is therefore produced at the controller 25 for the flyback voltage detection. However the parasitic capacitor 23 and the resistor 22 cause a low pass filtering to the flyback voltage VF. Besides the on-time TON of the control signal VG and the discharge time TDS of the flyback voltage VF are short at light load, which causes the waveform distortion of the detecting voltage VDET shown in FIG. 2. A low flyback voltage is therefore detected. This drawback is the main object of the present invention to overcome. 